This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We are developing the delta-P1 and higher order diffusion based models to allow accurate processing of FDPM data in a broader number of clinical situations. Specifically, are making FDPM measurements to probe layered tissue structures as well as assess the properties of small tissue volumes. In both these applications, we require FDPM measurements to be made at small source-detector (S-D) separations. These delta-P1 and higher order diffusion based models accommodate spatially distributed collimated sources and provide accurate predictions at small s-d separations and over a broad range of albedo. We will compare these results to solutions derived using the standard diffusion approximation and experiment. We will also solve the new equations for steady and amplitude modulated collimated sources illuminating the surface of an infinite medium. Compare results with SODT and experiment. We are currently investigating the use of this theoretical models to solve th e in verse problem at small s-d separations.